Summary of Work: A low frequency (15%, 9/59) of H-ras codon 61 mutations was detected in hepatocellular neoplasms when compared to the higher frequency (59% of this study and 56% of historical data) in spontaneous liver neoplasms. There was no difference in the mutation frequency or spectrum among exposure groups or between benign and malignant heopatocellular neoplasms. K-ras mutations at codons 12, 13, and 61 and H-ras mutations at codon 117 were not detected in hepatocellular newplasms. The frequency appeared to be similar to those identified in liver neoplasms from the structurally related chemical tetrachloroethylene (TCE). These data suggest that TFE-induced heptocellular neoplasms from structurally related chemical classes most likely develop by a pathway independent of H- and K-ras proto- oncogene activation. The manuscript has been published in Toxicologic Pathology. Other tumor types including hemangiosarcomas of the liver in the TFE study will be evaluated for genetic alterations.